The functional properties of neurons in the mammalian cerebral cortex depend on the selective termination of afferent axons and the stereotyped dendritic morphology of the neurons, themselves. An interaction between afferents and neurons appears to regulate the morphology of cortical cells, such that alterations in afferent distribution produces changes in the shape and extent of dendritic fields and, through them, changes in the physiological properties of cortical neurons. In this project, the proposed studies will examine the normal development of neurons in the rat first somatic sensory cortex (SI) and primary visual cortex (V1) and determine how manipulations of the sensory periphery change the normal developmental processes. To accomplish these tasks, the fixed-slice preparation will be used to identify, inject, examine and classify neurons of the rodent cerebral cortex. The experimental strategy is made up of three parts: 1) Identification of neurons in the aldehyde fixed slice. Neurons will be selected in the slice preparation either by their position alone, which will require determination of laminar borders in rat V1 and barrel borders in rat SI, or by their position plus the expression of cell surface markers which allows specific classes of cells to be targeted. The pre-selected neurons will then be injected intracellularly so as to reveal their entire dendritic fields. 2) Detailed light and electron microscopic examination of intracellularly injected cortical neurons. Injected neurons will be reconstructed with a laser confocal microscope to follow the pattern of morphological development of large neuronal populations. Histochemical methods will be used to convert the injected dye into an electron dense reaction product so that the development of synaptic patterns, particularly the innervation of cortical neurons by thalamocortical axons, can be traced. Both normal animals and animals that have undergone manipulations of the sensory periphery will be examined so that the response of cortical neurons to an altered afferent innervation can be determined. 3) Chemical characterization of the injected neurons. Immunocytochemical methods will be used to simultaneously identify the injected neurons and to characterize them as belonging to a particular neurotransmitter-specific or other neurochemically-characterized neuronal class. These studies address one of the fundamental aspects of cortical development, that of the process whereby the morphology of individual neurons and of entire neuronal populations develops and changes in response to events that occur in the sensory periphery. They are relevant to situations, including genetic defects and environmental perturbations such as the introduction of toxins, which may change the morphological characteristics and physiological properties of cortical cells.